Well, I don't know about Win2K, but on XP, if you open the properties of the dial-up connection, there is a page with options where you can specify the idle time after which it automatically hangs up. The smallest amount you can choose is one minute, though.
I'd rather suggest to look for another provider with a volume rate. Your current one sounds quite expensive if every second offline time counts. Even back in the times when I used a serial modem and telephone costs where rather high, I never minded being online for half an hour or so. I cannot imagine that DSL should be so much more expensive nowadays.
Bye,
Thomas